1. Field of the Invention
The present invention relates to a sheet workpiece position device specifically designed for moving, at high speeds, of various sizes to serve fixed heads for hemming and/or cutting the sheet workpiece.
2. Description of the prior art
For the hemming and cutting of the sheet workpiece, use is made both of mechanical machine tools (hemming machines, nibbling machines) and thermal machine tools (based on plasma-arc cutting, laser cutting.
In processing a sheet workpiece along a cutting trajectory, in many cases, it is more advantageous to move the metal sheet workpiece with respect to a tool carrier head of the machine tool rather than to move the tool-carrier head of the machine tool with respect to the sheet workpiece fixed at standstill on a frame of the machine tool. This operation of positioning of the sheet workpiece with respect to the tool-carrier head is normally performed by gripping the sheet workpiece to be machined (usually rectangular in shape and of any size within the maximum trade formats) by means of a set of pliers. That is to say, the set of pliers grip the sheet firmly along one side and move it close to the machining head in a vertical position in machine tools with vertical or subvertical feed. In the machine tools with horizontal feed, the pliers move the workpiece close to the machining head on a roller table set around the head to prevent excessive bending of the sheet workpiece.
Although this method has been extremely useful in the past, now, because of the increase in the speed and acceleration of the movements currently set on handling devices, thanks to the progress made in electronic controller systems and in the design of servomechanisms, it is no longer practicable. That is to say, when this type of gripping technique is used, particularly on a thin sheet workpiece, it is difficult to control the condition of the workpiece, which, at high speeds, tends to split and "fly off", in the areas on the opposite side to the gripped side and along the lateral edges.
In addition, the pliers constituting gripping devices currently in use have a frame with opposing lever fulcrums, in which one of the jaws is fixed and the other is hinged to it and driven by a pneumatic (or hydraulic) actuator which produces the rotation required to open or close the jaws. This frame is rather bulky and, if relatively long jaws are required, it is extremely unreliable as its rigidity is considerably reduced. Further, this frame does not provide a great gripping strength and thus leads to reduced specific pressures on the sheet workpiece, so that the sheet workpiece handled (stainless steel, shot-blasted, prepainted, etc.) may come off from the griping devices to be severely damaged.